The Sunday Age's state political editor.

A rail link to Melbourne Airport would require the state government to build through several contaminated sites, acquire private land, and threaten environmentally sensitive areas - including some of ''national and state significance''.

One week after Premier Denis Napthine announced that a long-awaited train line to Tullamarine would become a reality if he is re-elected in November, the government's own study has revealed the project could face a range of problems unless it is carefully managed.

Under the Coalition's plan, a new electrified rail service would run along dedicated tracks from Melbourne Airport to Albion, in the city's west, before joining the existing rail line to Southern Cross Station, picking up passengers every 10 minutes during peak times.

But according to an independent study commissioned by Public Transport Victoria, an airport rail link along this route could result in ''potential environmental constraints''; ''likely negative perceptions due to land acquisitions''; and ''implementation risks'' from the construction of a new rail flyover at Albion. The 170-page study, released last year, also confirms the airport link is largely dependent on the so-called Melbourne Metro rail project, a $10 billion underground cross-city tunnel designed take on thousands of extra passengers and connect the Dandenong and Sunshine rail corridors. ''The [Melbourne Metro] must be constructed prior to the opening of the Albion east option, as this option uses the tunnel to connect Melbourne Airport with the south-eastern suburbs of Melbourne,'' it says.

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The report, by consultant Parsons Brinckerhoff, also found:

■ The airport rail link would require land acquisition around Albion junction and Tullamarine, which could cause short-term disruption for commercial properties. (The government says no residential homes will be affected.)

■ The project would cut through about 670 metres of endangered vegetation and threaten biosites (which contain endangered flora, fauna or habitat) of state or national significance.

■ The link would also pass six contaminated land sites, ranging from high to low risk.

The idea of an airport connection between the city and Tullamarine has been on the books for more than 40 years, but Dr Napthine - who announced the election policy at last week's Liberal state council - has not told voters how much it would cost, when it will be complete, or what the likely ticket prices might be, insisting all will be revealed in the May 6 budget.

Questions also remain about how far the route has strayed from the original version, which Transport Minister Terry Mulder said last April would run from the airport to Albion using ''the existing rail tracks from Sunbury within the Sunshine corridor, and connect with the government's planned Melbourne Metro rail tunnel''.

Last week, Dr Napthine said the rail link ''will go from Southern Cross to Albion along the Jacana rail corridor and directly to the airport'' but refused to answer questions about how it would connect to the Melbourne Metro underground rail tunnel, fuelling speculation the project has been realigned.

Opposition transport spokeswoman Jill Hennessy said Victorians should be sceptical about the government's plan. ''Labor is not saying that an airport rail link is not a worthy project, however, the services that people use every single day are our priority,'' she said.

Experts have also warned it would be cheaper and easier to build once the Metro tunnel and the Regional Rail Link was in place. Public Transport Victoria chairman Ian Dobbs said last year: ''Regional Rail Link is being built and it will be ready in the next couple of years. The second stage, which is really the Metro tunnel, is the No. 1 project on Infrastructure Australia's list … If you're going to fund anything, this should be your first priority.''

Airshuttle Commuter Express director Peter O'Brien, who has proposed building a $1 billion monorail to the airport, said the government could save billions if it was not so hung up on heavy rail.